The invention relates to a method and an apparatus for picking up sound.
In a hearing aid, sound is picked up, amplified and at in end transformed to sound again. In most cases omnidirectional microphones are used for picking up sound. However, in the case of omnidirectional microphones, the problem occurs that ambient noise is picked up in the same way. It is known to enhance the quality of signal transmission by processing a signal picked up by the hearing aid. For example, it is known to split the signal into a certain number of frequency bands and to amplify preferably those frequency ranges in which the useful information (for example speech) is contained and to suppress those frequency ranges in which usually ambient noise is contained. Such signal processing is very effective if the frequency of ambient noise is different from the typical frequencies of speech. There is little help in the so-called “party situation”, in which the useful signal is speech of one person and noise consists of speech of a lot of other people. To overcome this problem it has been proposed to use directional microphones with a cardioid or hyper-cardioid characteristic. In such cases sound of sources in front of the person wearing the hearing aid is amplified and sound from other directions is suppressed. Directional microphones are often used in these situations, but they have several serious disadvantages. For instance, the directional microphones are bulky, usually have higher equivalent input noise, and are extremely sensitive to wind. The situation becomes even more problematic when stereo or surround record is required. Then, it is necessary to use more microphones. U.S. Pat. No. 5,214,709 teaches that usually pressure gradient microphones are used to pick up the sound at two points with a certain distance to obtain a directional recording pattern. The largest disadvantage of the simple small directional microphones is that they measure air velocity, not sound pressure, therefore their frequency response for the sound pressure has a +6 dB/octave slope. This means that their pressure sensitivity in the range of low frequencies is much lower than at high frequencies. If inverse filtering is applied the microphone's own noise is also amplified on the low frequencies and the signal to noise ratio remains as bad as it was before the filtering. The second problem is that if the directional microphone is realized with two omnidirectional pressure microphones, their matching is critical and their frequency characteristic depends very much on the incoming sound direction. Therefore, the inverse filtering is not recommended and can have a negative effect. Because of the mentioned reasons omnidirectional pressure microphones with linear frequency response and a good signal to microphone noise ratio on whole frequency range are mostly used for peaceful and silent environments. When the noise level is high, the directionality is introduced, and since the signal level is high, the signal to microphone noise ratio is not important.
Furthermore, U.S. Pat. No. 5,214,907 describes a hearing aid which can be continuously regulated between an omnidirectional characteristic and a unidirectional characteristic. The special advantage of this solution is that at least in the omnidirectional mode a linear frequency response can be obtained.
It is further known from M. Hackl, H. A. Müller: Taschenbuch der technischen Akustik, Springer 1959 to use double membrane systems for obtaining a directional recording pattern. Such systems are used in studios and professional applications. However, due to losses caused by membrane mass and friction the real capabilities are partially limited. It is not known to use such systems for hearing aids.
Some documents, e.g., EP 690 657 A, EP 869 697 A or U.S. Pat. No. 3,109,066 a disclose microphone systems in which the signals of microphones are delayed and these delayed signals are mixed with original signals of the microphones. In that way a cardioide pattern can be obtained for example. However, such feed forward solutions show a frequency response for the sound pressure having a +6 dB/octave slope. Generally this disadvantage can be partially overcome by selective amplification of the signals, but then noise is amplified too and the signal/noise ratio is deteriorated.